Recently, there is dramatically growing demand for portable electronic products such as laptop computers, video cameras and mobile phones, and with the extensive development of electric vehicle, accumulators for energy storage, robots and satellites, many studies are being made on high performance secondary batteries that can be recharged repeatedly. Particularly, lithium secondary batteries are gaining more attention than nickel-based secondary batteries because of their advantages; lithium secondary batteries have little or no memory effects so that they can be freely charged and discharged, and have a very low self-discharge rate and high energy density.
A lithium secondary battery mainly uses lithium-based oxide and carbon material for a positive electrode active material and a negative electrode active material, respectively. The lithium secondary battery includes an electrode assembly composed of a positive electrode plate and a negative electrode plate respectively coated with the positive electrode active material and the negative electrode active material, with a separator interposed between, and a packaging, also known as a battery case in which the electrode assembly and an electrolyte solution are received and which is hermetically sealed.
More recently, secondary batteries are widely used in not only small-sized devices such as mobile electronic devices, but also medium- and large-sized devices such as vehicles or energy storage systems. Particularly, with the gradual depletion of carbon energy and the increasing interest in environment, the demand for hybrid electric vehicles and electric vehicles is increasing all over the world, including US, Europe, Japan, and Korea. The most essential component of a hybrid electric vehicle or electric vehicle is a battery pack which gives a driving force to a vehicle motor. Since hybrid electric vehicles or electric vehicles obtain a driving force through charging/discharging of the battery pack, they have higher fuel efficiency than vehicles using only an engine, and have little or no pollutant emission. That is the reason why the number of users is increasing. Furthermore, the battery pack of hybrid electric vehicles or electric vehicles includes multiple secondary batteries, and the multiple secondary batteries are connected in series and in parallel to improve the capacity and output.
Vehicles using electrical energy are directly influenced by the performance of the battery pack, requiring a battery management system (BMS) that efficiently manages the charge/discharge of each secondary battery by measuring the voltage of each secondary battery and the voltage and current of all the secondary batteries, and detects any degraded or failed secondary battery from the secondary batteries so that each secondary battery has the maximum performance.
Hybrid electric vehicles or electric vehicles have a large capacity battery pack mounted thereon to supply sufficient energy necessary for the system, bringing properties of high voltage and high current. Accordingly, safety is most important of all. Particularly, the high capacity battery pack has risks of fire and explosion caused by over-charging and over-current, and in the event of a fire caused by failure and malfunction of the BMS which controls the battery pack, the safety of the vehicle and the system will be noticeably threatened.
To eliminate the risks, conventionally, fires were detected using a temperature sensor and a current/voltage sensor, and a fire extinguisher provided separately from the battery pack brought into action when a fire was detected. However, the conventional fire detecting and extinguishing methods need a temperature sensor and a current/voltage sensor, and these sensors lack accuracy in diagnosing fires at the early stage, often failing to protect vehicles and people, and especially during self-reaction, the role of the BMS is insignificant, causing damage to vehicles and people.